APPENDIX B

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narily be extremely slow. It is possible therefore to wipe off the
end of the capillary, put the weighed container in place, admit
the pressure for a known interval of time, touch off into the
container any material still adhering to the capillary and weigh.
From the weight of material, the volume of flow may be cal-
culated from the density when desired.
There is however, another convenient method which can be
used when the material comes from the capillary in drops.
The observer turns on the pressure and simply takes the time of
formation of a convenient number of drops, making no weighing
at all. Other measurements are made at the same or other
pressures. Finally without cleaning off the end of the capillary
a certain number of drops are counted off into a weighed receiver
at the minimum pressure used and also at the maximum pressure
used. From the weight of a drop at these two pressures, one can
calculate the weight of a drop at any intermediate pressure pro-
vided the weight is a linear function of the pressure. By this
method a large number of measurements on a given material can
be completed in a single day with an accuracy of 0.3 per cent.
According to measurements by H. D. Bruce the weight of the
drop is not always uniform at a given pressure.
The pressure pi delivered to the plastometer is calculated in
the manner already described (page 299 et seq.}, correcting for the
temperature of the liquid in the manometer. The plastic
material exerts a hydrostatic head which must be corrected for
as follows.
The initial head in the container, h, may be measured by the
use of a straight, slender wire. To this is added the length of
the capillary, I, hence the pressure (h + l)p added to pi, gives
the corrected pressure p to be used in calculating the plasticity.
The change of hydrostatic head in subsequent determinations
may be ascertained by noting the volume of plastic material
which has accumulated in the graduated receiver. In this case
it is also necessary to know how much the level of the material
in the container is lowered by the loss of 1 ml. A much better
plan is to have a graduated glass tube of just the size to fit into
the container, and open at both ends, cemented into the container.
Having cut away portions of the metal of the container, the level
of the material within may be read directly.
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